OMNI-Max Anchors Research Articles

Dynamic installation, keying and diving of OMNI-Max anchors in clay

OMNI-Max anchors are designed with the aim of diving upon pullout, gaining capacity with increasing pullout distance. However, this diving potential is yet to be confirmed; that is, the critical question to be resolved is whether or not the anchor dives upon pullout and, if so, in what conditions. This paper reports the results from three-dimensional dynamic finite-element analysis undertaken to provide insight into the behaviour of OMNI-Max anchors during pullout in non-homogeneous clay. Large-deformation finite-element (LDFE) analyses were carried out using the coupled Eulerian–Lagrangian approach, modifying the simple elastic–perfectly plastic Tresca soil model to allow strain softening, and to incorporate strain-rate dependency of the shear strength. The keying process and diving patterns during anchor pullout revealed three governing factors including: (a) pullout inclination angle; (b) padeye offset ratio; (c) final installation depth. For designing anchors to dive into deeper, it was recommended that the anchor loading angle to the horizontal at the padeye should be 45° or less and the anchor padeye offset ratio should be in the range of 0·25 ∼ 0·53 (offset angle of 14 ∼ 28°).

Numerical investigation on the penetration of gravity installed anchors by a coupled Eulerian–Lagrangian approach

Gravity installed anchors (GIAs) are released from a height of 30–150m above the seabed, achieving velocities up to 19–35m/s at the seabed, and embed to depths of 1.0–2.4 times the anchor length. Challenges associated with GIAs include the prediction of anchor initial embedment depth, which determines the holding capacity of the anchor. Based on the coupled Eulerian–Lagrangian approach, a numerical framework is proposed in this paper to predict the embedment depth of GIAs, considering the effects of soil strain rate, soil strain-softening and hydrodynamic drag (modeled using a concentrated force), with the anchor-soil friction described appropriately. GIAs are influenced by the hydrodynamic drag before penetrating into the soil completely, hence the anchor accelerates less than the previous investigations in shallow penetration, even decelerates directly at the terminal impact velocity. The hydrodynamic drag has more influence on OMNI-Max anchors (with an error of ∼4.5%) than torpedo anchors, and the effect becomes more significant with increasing impact velocity. An extensive parametric study is carried out by varying the impact velocity, strain rate and strain-softening parameters, frictional coefficient, and soil undrained shear strength. It is concluded that the dominant factor affecting the penetration is the soil undrained shear strength, then are the impact velocity, strain rate dependency and frictional coefficient, and the minimal is the strain-softening of soil. In addition, although the strain rate dependency is partly compensated by the softening, the anchor embedment depth accounting for the effects of strain rate and strain-softening is lower than that for ideal Tresca soil. Strain rate dependency dominates the combined effects of strain rate and strain-softening in the dynamic installation of GIAs, on which should pay more attention, especially for the calibration of the related parameters and the measured solutions. In the end, the theoretical model based on the bearing resistance method is extended by accounting for the hydrodynamic drag effect.

An efficient approach to incorporate anchor line effects into the coupled Eulerian–Lagrangian analysis of comprehensive anchor behaviors

With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated and significantly affected by the anchor line. Based on the coupled Eulerian–Lagrangian (CEL) method, a numerical approach incorporating anchor line effects is developed to investigate comprehensive anchor behaviors in the soil, including penetration of drag anchors, keying of suction embedded plate anchors and diving of gravity installed anchors. Compared to the method directly incorporating the anchor line into the CEL analysis, the proposed method is computationally efficient. To examine the robustness and accuracy of the proposed method, numerical probe tests and then comparative studies are carried out. It is found that the penetration (or diving) and keying behaviors of anchors can be well simulated. A parametric study is also undertaken to quantify the effects of various factors on the behavior of OMNI-Max anchors, whose mechanisms are not yet fully understood. The maximum embedment loss of OMNI-Max anchors during keying is not influenced by the initial anchor embedment depth, whereas significantly increases with increasing drag angle at the embedment point. With decreasing initial anchor embedment depth or increasing soil strength gradient, drag angle at the embedment point and diameter of the anchor line, the behavior of OMNI-Max anchors could change from diving to pullout, which is undesirable in offshore engineering practice. If the drag angle increases over a certain limit, the anchor will fail similar to a suction anchor.

Dynamic installation of OMNI-Max anchors in clay: numerical analysis

This paper reports the results from three-dimensional dynamic finite-element analysis undertaken to provide insight into the behaviour of OMNI-Max™ anchors during dynamic installation in non-homogeneous clay. The large-deformation finite-element analyses were carried out using the coupled Eulerian–Lagrangian approach, modifying the simple elastic–perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength using the Herschel–Bulkley model. The results were validated against field data prior to undertaking a detailed parametric study, exploring the relevant range of parameters in terms of anchor mass, impact velocity and soil strength. To predict the embedment depth in the field, an improved rational analytical embedment model, based on the total energy method, was proposed, with the large-deformation finite-element data used to calibrate the model.

Dynamic installation of OMNI-Max anchors in clay: numerical analysis

Dynamic installation of OMNI-Max anchors in clay: numerical analysis